Super-resolution formation fluid imaging data acquisition and processing

ABSTRACT

Cross-well electromagnetic (EM) imaging is performed using high-power pulsed magnetic field sources, time-domain signal acquisition, low-noise magnetic field sensors, spatial oversampling and super-resolution image enhancement and injected magnetic nanofluids. The acquired signals are processed and inter-well images are generated mapping electromagnetic (EM) signal speed (group velocity) rather than conductivity maps. EM velocity maps with improved resolution for both native and injected fluids are provided.

CROSS REFERENCE TO RELATED APPLICATIONS

The present application is a continuation of, and claims priority to,co-pending, commonly owned U.S. patent application Ser. No. 13/707,721,filed Dec. 7, 2012, and to its related U.S. Provisional Application No.61/568,403, filed Dec. 8, 2011.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to imaging sub-surface structures,particularly hydrocarbon reservoirs and fluids therein, and moreparticularly to cross-well and borehole-to-surface electromagnetic (EM)surveying.

2. Description of the Related Art

Cross-well and borehole-to-surface electromagnetic (EM) surveying haveinvolved continuous-wave (CW) EM sources placed in one borehole andreceivers/sensors which detected the phase and amplitude of the EMsignal in a distal borehole, using multiple source and receiverpositions. The data readings were used to generate a synthetic timedomain version of the signal, and inferred transit times were used alongwith source/receiver geometry to create a 2D conductivity matrix orimage of the inter-well plane via inversion with ray-tracing.

Brine which is electrically conductive is everpresent in hydrocarbonreservoirs, and the presence of brine attenuated EM signals inproportion to their frequency. The presence of brine, as well as thelarge inter-well distances on the order of 1 kilometer or more, andthermal noise limits in conventional receivers caused continuous-wave EMsurveys to require very low frequency operation, usually about 200 Hz.The low frequency operating range which was required severely limitedcross-well imaging resolution as it: a) is in a diffusive regime, and b)had a very large wavelength. At present, so far as is known, a spatialresolution of only 1/10th to 1/20th of the inter-well spacing has beenobtainable.

Since practical spacing for boreholes in hydrocarbon reservoirs usuallyspan hundreds to thousands of meters and such reservoirs are usuallyassociated with electrically conductive brines, significant EM signalattenuation across a reservoir has been encountered. Such attenuation isfrequency-dependent, such that higher frequencies are attenuated morethan lower frequencies. Since higher frequencies have shorterwavelengths, and therefore afford better imaging resolution, it would beadvantageous to operate at the highest frequency that still gives adetectable signal after transiting the reservoir region of interest.However, the presence, concentration and distribution of brines aregenerally unknown prior to investigation, and the optimal frequency forinvestigating the reservoir with EM surveying could not be determined inadvance.

SUMMARY OF THE INVENTION

Briefly, the present invention provides a new and improved apparatus forelectromagnetic imaging of a subsurface hydrocarbon reservoir. Theapparatus includes an electromagnetic energy source emitting pulses ofelectromagnetic energy to travel through the subsurface hydrocarbonreservoir. A plurality of electromagnetic sensors in the apparatus formsa measure of the arrival time of the emitted pulses from theelectromagnetic energy source. The apparatus also includes a processorwhich analyzes the measure of arrival time data from the plurality ofelectromagnetic sensors to form a representation of subsurface featuresof the subsurface hydrocarbon reservoir. A display in the apparatusforms an image of the representation of subsurface features of thesubsurface hydrocarbon reservoir.

The present invention also provides a new and improved method ofelectromagnetic imaging of a subsurface hydrocarbon reservoir. Pulses ofelectromagnetic energy are emitted to travel through the subsurfacehydrocarbon reservoir, and a measure of the arrival time of the emittedpulses at a plurality of electromagnetic sensors is formed. The measureof arrival time data from the plurality of electromagnetic sensors isanalyzed to form a representation of subsurface features of thesubsurface hydrocarbon reservoir, and an image of the representation ofsubsurface features of the subsurface hydrocarbon reservoir is thenformed.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram of a transmitter-receiver array for aborehole to surface electromagnetic survey.

FIG. 2 is a schematic diagram of a transmitter-receiver array for aborehole to borehole electromagnetic survey.

FIG. 3A is a schematic diagram of a transmitter of electromagneticenergy for an electromagnetic survey.

FIG. 3B is a schematic diagram of a receiver of electromagnetic energyfor an electromagnetic survey.

FIG. 4 is a plot of a power spectrum for a square wave electromagneticenergy signal.

FIG. 5A is a schematic electrical circuit diagram of a pulse generatorfor an electromagnetic survey.

FIG. 5B is an example waveform of a pulse generated by the pulsegenerator of FIG. 5A.

FIG. 5C is an example waveform of an actual pulse generated by a pulsegenerator for an electromagnetic survey.

FIG. 6A is a schematic electrical circuit diagram of a semiconductorbased pulse generator for an electromagnetic survey.

FIG. 6B is a plot of an example voltage and current waveform generatedby the pulse generator of FIG. 6A.

FIG. 7 is a schematic electrical circuit diagram of an equivalentcircuit for an induction sensor for an electromagnetic survey.

FIG. 8 is a schematic diagram of an example borehole to boreholeelectromagnetic survey according to the present invention.

FIGS. 9A, 9B and 9C are plots of range versus power for variousfrequencies and conductivities of subsurface media.

FIG. 10 is a schematic diagram of another borehole to boreholeelectromagnetic survey according to the present invention.

FIG. 11 is a schematic diagram of another borehole to boreholeelectromagnetic survey according to the present invention.

FIG. 12 is a schematic diagram of another borehole to boreholeelectromagnetic survey according to the present invention.

FIG. 13 is a schematic diagram of test results from a borehole toborehole electromagnetic survey according to the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

By way of introduction, the present invention involves imagingsub-surface structures, particularly hydrocarbon reservoirs and fluidstherein. The primary approach is related to cross-well andborehole-to-surface electromagnetic (EM) survey technology. The presentinvention specifically focuses on fully time-domain data acquisitionusing high-power pulsed EM sources. The present invention can alsoinclude spatial over-sampling and super-resolution data processingtechnology to improve image resolution. The present invention can alsoutilize magnetic materials to provide image contrast for regionscontaining injected fluids.

An improved approach to cross-well EM imaging is provided, using acombination of high-power pulsed magnetic field sources, fullytime-domain signal acquisition, modern low-noise magnetic field sensors,spatial oversampling and super-resolution image enhancement and injectedmagnetic nanofluids. The approach provided by the present inventiongenerates inter-well images mapping EM signal speed (group velocity)rather than conductivity maps. Conventional continuous wave (CW) sourcesare typically limited to about 1500 watts in the down-hole environment.In contrast, pulsed sources according to the present inventionfacilitate simple time-of-arrival data acquisition schemes and readilysupport megawatt transmitters. As will be described, simple current looptransmit antennas can be driven from fast-discharge energy sources(capacitors) through Blumleins, Marx generators, simple spark gaps,pulse forming LC networks or other sources to generate the requisitecurrent and power levels. Higher power levels increase range and/oroperating frequency in cross-well EM imaging. Using such sources andavailable modern magnetic field sensors (such flux gates, SQUIDs, searchcoils and the like) with noise figures in the pico- to femto-Tesla perHz range, substantial improvement in the P/N ratio (transmit power toreceiver thermal noise) are available compared to the prior art. Sincethe hydrocarbon reservoir fluid structure and composition changes onlyslowly, time is available to perform such measurements with relativelysmall transmitter/receiver positioning steps. This ‘oversampling’ isused with super-resolution image deconvolution methods to improve imageresolution by a factor of three to ten, depending on the amount ofover-sampling performed. Finally, fluids (typically water) loaded withmagnetic nanoparticles can be injected, which readily reduces groupvelocity by five to ten percent compared to pure water. This fluid canbe imaged against a background of native or previously injected water.In a water-flood environment, this is useful to determine dynamic flowpaths of injected fluids. Accordingly, the present invention provides EMvelocity maps with up a resolution of up to 100^(th) (i.e., 5 to 10times greater than previously available) of the inter-well spacing forboth native and injected fluids.

With the present invention an electromagnetic (EM) pulse with knowncharacteristics is generated from a high power, pulsed electromagneticpulsed EM source at one location in or near subsurface reservoir. Theemitted pulsed EM signal is transmitted through the reservoir andrecorded at one or more other EM energy receivers after travel throughthe subsurface formations of the reservoir. The EM signal recorded aftertransiting the reservoir differs from the transmitted signal incharacteristics (e.g. time, amplitude, power spectrum, etc.) that dependon the properties of the intervening medium (e.g. the reservoir) andspatial variations of those properties.

In FIG. 1, an example arrangement of EM survey locations for a sourcearray 20 of transmitters Tx disposed in a well bore or borehole 22 toemit EM energy. In FIG. 1, a suitable number of arrays 24 of EM energyreceivers Rx are disposed on the earthen surface forming what is knownas a borehole to surface array. As also shown in FIG. 1, another set orarray 26 of EM energy receivers Rx is disposed in another well borehole28 spaced from the transmit borehole 22.

Transmitters Tx may be placed within a borehole or at the surface of theterrain. Likewise, receivers Rx may be placed within a borehole or atthe surface of the terrain. More than one borehole may be employed; andsuch a configuration is generally called ‘cross-well’. If only oneborehole is employed in conjunction with a surface array, theconfiguration is generally called ‘borehole-to-surface’. Both of theseconfigurations are shown in FIG. 1. Generally, at least one borehole isemployed so that EM signals can traverse the region of interest

A multitude of EM energy measurements are performed with differentcombinations of transmitter and receiver locations in order to samplevarious parts of the reservoir from different directions, as shown inFIG. 2. In FIG. 2, a number of transmitters 32 in a transmit borehole 34emit high power pulsed EM energy to transit subsurface earth formationsto a set of receivers 36 in a receiver borehole 38. Waveform displayssuch as those shown at 40 a, 40 b and 40 c represent example readingsfrom receivers 36 at certain depths as functions of EM signal traveltime. Multiple measurements of transmissions such as those shown in FIG.2 may be summed or averaged at a given transmitter and receiver locationpair to improve signal to noise ratios. Multiple transmitters (e.g. anarray) may be employed, as well as multiple receivers (e.g. an array).Generally, transmitters and receivers, either individual or arraysthereof, are placed at multiple locations to sample various parts of thereservoir and sampling each part from different directions.

According to a preferred embodiment of the present invention loopantennas and pulsed currents transmitters generate high power EMsignals, as shown in FIGS. 3A and 3B. An example such transmitter 32(FIGS. 2 and 3A) includes a loop antenna 44 with a spark gap 46 isconnected to a power supply high voltage module 48. A capacitor 50 isconnected in the loop antenna 44 across the leads from the power supplymodule 48, and a load resistor 52 is connected between the power supplymodule 48 and the spark gap 46.

An example transmitter 32 of FIG. 3A is one of a number of such units asshown schematically in FIG. 2 and is mounted with a tool or sonde 54lowered by a supporting wireline 56 from a vehicle 58 at the surface.The transmitter 32 and other transmitters shown in FIG. 2 are moved to anumber of depths in a transmit wellbore during EM surveying. A systemcontrol unit 60 associated with the vehicle 58 at the surface sendssignals or pulses over the wireline as indicated at 62 to provide theenergy for the pulses being transmitted. Once enabled, the high voltagepower supply 48 charges up an energy storage capacitor 50 through acurrent limiting resistor 52 until it reaches the break-down voltage ofa spark-gap 46. The current pulse is discharged through the loop antenna44, which may have more than one turn or loop. A small current loop 45,or sense coil, is supplied to capture a small portion of the magneticfield generated by 44; the sense coil 45 is connected to a coaxial cable56 that is used to provide a start-signal to recording apparatus orinstrumentation. It should be understood that other forms of transmittermay be used, as well.

An example receiver 36 (FIGS. 2 and 3B) includes a loop antenna 66connected to a recording instrumentation or oscilloscope module or card68. The receiver 36 of FIG. 3B is one of a number of such units as shownschematically in FIG. 2 and is mounted with a tool or sonde 70 loweredby a supporting wireline 72 from a EM logging vehicle 74 at the surface.The receiver 64 and other receivers shown in FIG. 2 are moved to anumber of depths in a receive wellbore during EM surveying. Recordingand processing instrumentation associated with the EM logging vehicle 74at the surface, on command from surface recording and processinginstrumentation over ‘start bus’ in wireline 72, sends records of highenergy EM pulses back over the wireline 72 as received after transitthrough the reservoir of interest from the transmit wellbore. Therecords are then stored on computer 60 and available for furtherprocessing and computerized analysis. It should be understood that otherforms of receiver may also be used.

According to the present invention, generally square current pulses ofhigh energy EM energy and selectable length and rise-time are providedas the generated EM signal from the EM energy transmitters. Such EMpulses are advantageous because they are relatively simple to generateand control and contain a broad range of frequency components. FIG. 4shows a representative power spectrum for both a single (dotted envelope78) and repetitive (arrows 80 at f₀ and its odd harmonics) pulses of thetype emitted by transmitters according to the present invention.

Since practical spacing for boreholes in hydrocarbon reservoirs usuallyspan hundreds to thousands of meters, and further since such reservoirsare usually associated with electrically conductive brines, significantEM signal attenuation across the reservoir has almost universally beenencountered. Such attenuation is frequency-dependent, and thus higherfrequencies are attenuated more than lower frequencies. Since higherfrequencies have shorter wavelengths, and therefore afford betterimaging resolution, it has with the present invention been advantageousto operate at the highest frequency that still gives a detectable signalafter transiting the reservoir region of interest. Since the presence,concentration and distribution of brines are generally unknown prior toinvestigation, the optimal frequency for interrogating the reservoircannot be determined in advance. Therefore, according to the presentinvention an inherently broadband EM source has been utilized, as isprovided by the square pulsed loop antenna transmitter 32 shown in FIG.3A.

A further advantage provided with the present invention is the abilityto dynamically control the length of the emitted EM energy currentpulse. Reducing the length of the current pulse increases f₀ and pushesthe envelope towards higher frequencies, ensuring that the best possibleresolution is obtained when imaging the reservoir at a given T-R spacingand signal power. Another useful feature of the signal shape utilizedwith the present invention occurs as a result of the frequency spectrumof the transmitted energy includes significant power at 0 Hz. This hasvalue in conjunction with injected magnetic nanofluids, as will bedescribed below.

A number of EM sources of a several conventional types may be employedas long as such a source includes some time-varying feature that can beused to determine travel-time across the reservoir and it has enoughpower to allow detection at the receiver location. The preferred sourceincludes a loop antenna like that shown schematically at 32 in FIG. 3A,which may have multiple conductor turns. This antenna is preferablydriven by a pulsed high current square wave, as has been described. Thiscurrent profile may be conveniently generated by a pulse forming circuitsuch as the type known in the art as a Blumlein circuit or a type knownas a thyristor circuit. It should be understood that suitable circuitsand sources may of course be used.

An example Blumlein source is illustrated schematically at 82 in FIG.5A. When a switch 84 is closed, an ideally rectangular pulse 85 as shownin FIG. 5B is applied across a load 86 that has the same impedance asthe pulse generator 82. The amplitude of the pulse 85 shown in FIG. 5Bis determined by the charging voltage and the pulse duration isdetermined by the length of the transmission lines and the propagationvelocity, v, of the electrical signal. FIG. 5C is a plot of a singleoutput pulse 88 from the Blumlein pulse generator 82 with a 10-ns timeduration and an amplitude of 35 kV into a 10 ohm load. A pressurizedspark gap provides a rise time of 1 ns.

The Blumlein source 82 typically employs a spark gap (although someversions may be triggered externally) to initiate the pulse. Energy issupplied by a high voltage power supply 90, and energy for the pulse forthe purpose of EM surveying according to the present invention is storedin a high voltage coaxial cable or similar waveguide structure like thatshown at 92 in FIG. 5A. The length of the EM surveying pulse isdetermined by the length of the coaxial cable 92 and the characteristicimpedance and capacitance per unit length of the cable determines thedelivered current, which arrives at a nominally constant rate. Sincehydrocarbon reservoirs are often on the order of 10,000 feet below thesurface, a coaxial cable delivering high voltage to an EM source in aborehole can conveniently serve as a Blumlein source like that shown inFIG. 5A and generate pulses on the order of 10 microseconds in length.

An alternative power supply for the EM source can be comprised of acapacitor bank and a high voltage switch, usually a thyristor device, asshown schematically at 94 (FIG. 6A). A set of waveforms 96 and 98 (FIG.6B) illustrate voltage and current, respectively, for a maximum currentpulse in accordance with the present invention. The term ‘thyristor’ isused to identify a class of closely related semiconductor devicesincluding a SCR (silicon controlled rectifier) and an IGBT (insulatedgate bipolar transistor). Such semiconductor devices deliver longerpulses where the length of a Blumlein source pulse would not besuitable. Such solid-state switches also offer lower maintenance andlonger useful lifetimes compared to mechanical or gas dischargeswitches, with modest increase in cost and complexity. It should benoted that a thyristor could be used to switch the output of a Blumleinpulse generator, although the more common configuration is to use acapacitor bank for energy storage, and to limit or regulate the currentoutput by selecting or controlling the impedance of the load device.

Receivers 36 used in the present invention preferably take the form of amagnetic field transducer and a time-domain recording device. Therecording device may be as simple as a zero-crossing discriminator and afast counter for recording the signal arrival time, or a more complexand costly transient recorder or digital oscilloscope module may beemployed. It should be understood that a may be used. A simple andgenerally suitable transducer is a loop antenna, as depicted in FIG. 3Bat 66 representing an example of a magnetic field sensor known as a‘search coil’. Search coils generally have many conductor turns toincrease sensitivity.

It should be understood that a wide variety of devices may be used tocapture the magnetic field generated by the EM source 42 and convert thesensed magnetic field readings into electrical signals that may berecorded for later analysis. Alternative sensors may includesuperconducting devices known as SQUIDs, fluxgates, Hall sensors andspin-valves. An exemplary search coil sensor is shown schematically inequivalent circuit form in FIG. 7 at 100.

Spatial and Temporal Oversampling

Super-resolution image enhancement comprises a further aspect of thepresent invention. Imaging resolution is often considered to be boundedby the diffraction limited resolution based on the wavelength of the EMprobe. However, spatial oversampling can be performed in conjunctionwith an inversion model that includes some knowledge of boundaries andstructure of the system. This is done to generate images with resolutionwell beyond what would normally be considered to be the diffractionlimit—especially if the sampling is performed in the near-field.

A ready example comes from induction logging in the oil field. Inductionlogging often operates at a frequency around 1 MHz, which would indicatea wavelength and resolution on the order of 100 to 1000 meters,depending on the impedance of the formation. In practice, however, withappropriate inversion code and knowledge of the response of layeredmodels of rock formations, useful resolution on the order of 1 meter orless is routinely achieved.

With the present invention, cross well EM imaging resolution of up to1/20^(th) of the interwell spacing is considered a practical anddesirable goal. Indeed, in published literature a signal source emittinga continuous wave signal at a frequency of about 200 Hz was used withinter-well spacing of 850 meters to generate images with about 45 meterblocks. A distance of 45 meters is a miniscule fraction of thewavelength of a 200 Hz EM signal (˜1000 m in a medium with 0.05 Siemensconductivity). Thus, imaging resolution depends on sampling frequencymuch more that it does on the wavelength of the EM probe.

In contrast, with the present invention air-coil antennas orientedperpendicular to the borehole axis are employed. Further, broadbandpulsed operation, and measurement spacing of 1 meter or less, especiallyaround the locus of fluid injection and production are used.

Data Processing

Tomographic inversion converts data acquired in the field into images ofthe reservoir. An example of this processing is described by Abubakar,et al. (“A fast and rigorous 2.5D inversion algorithm for cross-wellelectromagnetic data”, SEG Extended Abstracts, 2005 Annual Mtg. Houston,Tex.). The processing task requires solution of a full nonlinear inversescattering problem that is usually ill-conditioned and non-unique. Theirapproach employs a finite-difference code as a forward simulator,wherein the configuration is numerically discretized using a smallnumber of cells determined by the optimal grid technique. The forwardproblem is solved in each inversion step, and a LU decomposition methodis used to obtain a solution for all the transmitters simultaneously. Itis to be noted that the finite-difference approach so used in the priorart, based on coarse rectangular grid elements, introduces significantlimitations (simplifications) on the forward solution in order to speedup the computations.

A slightly different approach was described in U.S. Pat. No. 5,373,443.The approach used was based on a solenoid (coaxial with a borehole)source driven by a pure sine wave and recording the amplitude and phaseof the magnetic field at a distal borehole with another solenoid (alsocoaxial with the borehole). This measurement, called the diffusionfield, was transformed mathematically into a wave field, and then signalvelocity between source-receiver pairs was inferred from the wave field.These ‘rays’ were used to tomographically construct a conductivity mapof the inter-well region.

In contrast, with the present invention a pulsed broad band EM source isemployed, and the received waveform recorded in the time-domain. Thetravel times for each source-receiver pair are measured directly.Fourier transforms may then optionally be used to decompose the receivedsignals into their various frequency components and thereby extracttravel times as a function of frequency. The additional information thusmade available can be used to improve an inverted velocity image whenthe medium has material dispersion—since different frequency componentswill have different travel times and diffraction paths in a ray-tracingmodel.

In addition, variable density adaptive (triangular) mesh elements areemployed in a finite-element model for generating forward solutions,similar to the meshing approach used in COMSOL Multi-Physics. Thisapproach increases the mesh density close to the source and receiverregions. Since geologic models are normally coarse and blocky, at leasta dual grid paradigm is employed for generating velocity images of theregion of interest.

The objective according to the present invention is to detect andmonitor the path of injected fluids. Thus, as a furtherrefinement/embodiment a streamline model is employed for estimating thepath and volumes of injected fluids. Streamline simulators transform a3D block model into a number of flow paths of nominally constant flux.While the paths are inherently 3D, they can be solved independently asessentially one dimensional problems, increasing computationalefficiency tremendously. Since each streamline operates independently,they can also be treated as a quasi-orthogonal basis set for comprisingthe total flow of injected fluids.

Using the initial geology model and pre-injection EM data, a forwardsolution of the impact (change in EM field and propagation) of fluidinjected along each streamline can be computed independently. A linearsum of these components can then be determined that best fits theobserved EM field and travel times observed after injection of fluid fora period of time. The results can be mapped back onto the originalgeologic model to update fluid compositions as a function of time aswell as indicate appropriate porosity and permeability changes in theunderlying geologic model. This approach thus employs a tri-gridmodeling system: Cartesian blocks for the geologic model, streamlinesfor fluid flow and variable triangular meshes for EM transport in anquite different fashion form than conventional processing.

Contrast Imaging

Magnetic contrast enhancement provides a unique signature for injectedfluids. Oil, gas, water, brine and reservoir rocks generally haveessentially zero magnetic character. Another aspect of the presentinvention employs injected fluids to change the magnetic character ofthe reservoir volume invaded by such fluids. This may be accomplished byloading the injection fluid with pre-fabricated magnetic nanoparticlesor non-magnetic chemicals that can subsequently react when inside thereservoir to generate magnetic materials.

The group velocity of an EM signal depends on the dielectricpermittivity and magnetic permeability of the medium in a very simpleway: ν=(∈μ)^(−1/2). Thus, by injecting a fluid with μ=10 into aformation with 20% porosity, that reservoir volume will have aneffective magnetic permeability of 2, and the velocity of an EM wavetraversing it will decrease by about 30% (1/1.414). This time shift iseasily detectable with modern waveform recording instruments.

Transient Polarization & Relaxation of Magnetically Modified GeologicalStructures

Another aspect of the present invention involves observing delayedmagnetic transient response of magnetic materials in the reservoir afterinjecting fluids. It is to be noted that magnetic materials channelmagnetic flux in much the same way that good conductors channel electricfields and electric currents. Thus, the long-pulse nature of the EMsource employed with the present invention magnetically polarizesmodified regions of the reservoir in the vicinity of the EM source. EMenergy is converted and stored in the form of a static magnetic fieldwithin the modified portion of the reservoir. When the pulse ends, themagnetic field decays, possibly in a resonant fashion, withcharacteristics that depend on the magnetic permeability of the region,as well as the dimensions of the magnetized region. This magnetic fieldcan be detected both at the distal receivers (in another borehole or atthe surface), or alternatively back in the source borehole by using thesource antenna as a receiver.

The total magnetic energy stored in the modified region can be inferredfrom its B-field magnitude and temporal decay characteristics asobserved in the source region. Similarly, distal receivers will observea significantly increased B-field strength at frequencies correspondingto the RLC time constant of the magnetized reservoir volume. In a sense,the modified reservoir volume acts as a magnetic antenna, and moves theapparent EM source closer to the receivers. Give the exponential natureof attenuation in the reservoir, flood front anomalies generatesignificantly increased signal strength at the receivers. Detecting suchinvasive anomalies is another important aspect of the invention.

Another aspect of the present invention includes using a series ofmagnetic pulses of different lengths at a given source location.Locations with deeper fluid infiltration take longer to fully magnetize,and the depth of infiltration can be inferred from the time to magnetizethat region, as well as the length of the decay time as described abovewhen the source is switched off.

Another aspect of the present invention involves magnetization of thefluid modified reservoir volume from a distant EM source, which may bein another borehole, or may be situated at the ground surface. A surfacesource is particularly convenient because it may be moved around freelyand because removing the borehole geometric constraints, permits largerand more powerful EM sources.

In operation, a basic cross-well configuration shown in FIG. 8 isutilized in much the same way as depicted in FIG. 2. EM pulses aregenerated at each of the plurality of TX locations 122 indicated in afirst borehole 124, and the EM pulses are recorded at each of theplurality of RX locations 126 indicated in a second borehole 128. Thismatrix of observations is used to determine the travel time and signalstrength as a function of frequency between each of the TX-RX locationpairs. Inversion generates a 2D image of the EM velocity over thenominally planar surface containing the pair of nominally parallelboreholes. The EM velocity in water-filled rock is about four timesslower than in oil filled rock. The EM velocity of magnetically modifiedinjection fluid or magnetically modified reservoir volume can be fromfractionally slower to several times slower than water-filled rock.Conductivity and magnetic permeability tend to attenuate the EM signalin general, so the amplitude (or power) of the signal as a function offrequency indicates the average conductivity or product of conductivityand (magnetic) permeability along the line connecting a given TX-RXlocation pair. This information provides another constraint besidessimple group velocity and can be used during inversion to improve imagequality and accuracy. The process should be performed prior to injectionof fluids to capture the state and structure of the pristine hydrocarbonformation. The process is repeated periodically to image the progress ofthe flood front and/or modified reservoir volume as a function of time.

An important advantage to the broad-band pulsed (all the way down to 0Hz, FIG. 4) EM source used in the present invention essentially makescertain that some detectable signal from source to receiver is obtained,regardless of the distance and conductivity of the medium. Also madecertain is that the highest frequency that can traverse the distance andremain detectable is being generated and sampled. The present inventiontherefore provides the maximum signal and resolution (shortestwavelength) possible in a given field situation and configuration.

FIG. 8 is a diagram of an example EM surveying configuration accordingto the present invention. A simple pulsed source is formed based on aloop antenna, with a spark-gap trigger (which could be a thyratron,thyristor or comparable solid-state switch), and a Blumlein generatorcomprised of a long high-voltage coaxial cable connecting the surfacecomponents to the transmitter 120 in the borehole. Using a typical50-ohm coaxial cable a current pulse proportional in length to thelength of the cable (about 1 as per foot of cable) is generated. Thepower provided to the discharge and downstream components (e.g. the coilantenna) is determined by several parameters: the capacitance per footof the coaxial cable, group velocity, characteristic impedance andcharging voltage. The current is primarily limited by the characteristicimpedance of the coaxial cable, and the discharge power given by V*I.Using typical relationships for skin depth and attenuation of an EMsignal as a function of frequency and conductivity of the medium,reasonable ranges can be computed over which a detectable signal can beobserved using conventional components. A loop antenna, constructed as asearch coil, with a detection limit of about −100 dB is assumed as areceiver 125 in the distal borehole 128. In this simple arrangement thekey parameters are the charging voltage and the conductivity of themedium, and these factors determine the maximum frequency that isreadily detected at the distal borehole 128. Discharge power scales withthe square of the coaxial voltage. With a typical capacitance of 30 pFper foot, a charging voltage of 1,000 volts yields an output power ofabout 15,000 watts; 10 kV yields about 1.5 MW; 100 kV yields about 0.15GW. Plots of range vs. power under various assumptions of frequency andaverage conductivity of the medium detailed below in Tables I, II, andIII are set forth in FIGS. 9A, 9B and 9C, respectively.

TABLE I ANHYDRITE cpf 3.00E−11 capacitance per foot DL −100.00 dbdetection limit sigma 0.00005 S/m conductivity freq 1,000,000 Hzfrequency lambda 4.47E+02 m wavelength atten 0.1 db/m attenuation Z 50ohm impedance SD 73.57180382 m skin depth NormRange 1471.436076 20 SDrange

TABLE II OIL-FILLED ROCK cpf 3.00E−11 capacitance per foot DL −100.00 dbdetection limit sigma 0.005 S/m conductivity freq 50,000 Hz frequencylambda 2.00E+02 m wavelength atten 0.3 db/m attenuation Z 50 ohmimpedance SD 32.90231092 m skin depth NormRange 658.0462183 20 SD range

TABLE III RINE-FILLED ROCK cpf 3.00E−11 capacitance per foot DL −100.00db detection limit sigma 0.5 S/m conductivity freq 500 Hz frequencylambda 2.00E+02 m wavelength atten 0.3 db/m attenuation Z 50 ohmimpedance SD 32.90231092 m skin depth NormRange 658.0462183 20 SD range

Nominal conductivities for various materials include: anhydrite (FIG.9A): 0.00005 S/m; oil-filled rock (FIG. 9B): 0.005 S/m; and brine-filledrock (FIG. 9C): 0.5 S/m. The nominal conductivity of seawater is 5 S/m.FIGS. 9A, 9B and 9C present tables for anhydrite, oil-filled rock andbrine-filled rock, respectively, with operating frequencies detectableat 1 km.

In a commonly occurring situation (FIG. 10) where a reservoir isproduced using a combination of peripheral horizontal injector wells forwater flooding or pressure maintenance and vertical producer wellselsewhere in the reservoir, EM TX arrays 130 in an injector well 132 andRX arrays 134 in a producing well 136 are deployed as shown in FIG. 10.Other than a non-planar sample volume, operational details are likethose described in connection with FIG. 8 above. The configuration ofFIG. 10 is useful for visualizing progress of an oil-water contact lineor fluid front 138 over time while a formation 140 in the reservoir isproduced. This information is vital for optimizing reservoir management.

The configuration shown in FIG. 11 provides an improved projection ofthe oil-water contact (or flood front) compared to that of FIGS. 8 and10, and is especially useful in detecting flood-front anomalies thatarise from super-K zones or fracture corridors as indicated at 148. EMTX arrays 150 arranged over ground surface 152 and RX arrays 154 in ahorizontal injector well 156 are deployed as shown in FIG. 11. Avertical producer well is shown at 160. Production is being had from tworeservoirs 162 and 164, each with a fluid front as indicated at 162 aand 164 a, respectively. The configuration illustrated in FIG. 11 may beconveniently implemented where a given hydrocarbon reservoir hasmultiple producing horizons.

A further configuration, shown in FIG. 12, may be employed where onlyone borehole as indicated by a horizontal injector well 170 is availablefor data to be gathered. The well 170 contains one or more transmitters172 and is located in a hydrocarbon reservoir 174 shown in plan view inFIG. 12. A long duration pulse is emitted from transmitters in the well170 and is used to magnetize the injected fluids and/or the modifiedreservoir volume, and magnetic transients are observed along theborehole 170 containing the transmitter(s) 172. High permeabilityanomalies (super-K or fracture corridors 175) may be thereby detectedfrom the single borehole 170 by estimating the total magnetic energystored at each station along the borehole as indicated at 177 forseveral stations. A fluid front based on such estimates is schematicallyshown at 178. Regions with more injection fluid or greater modifiedreservoir volume display larger stored energies, and therefore higherremnant field strength and longer decay times. If a second distalborehole is available as shown at 176, RX stations may also be deployedand cross-well EM data gathered, similar to the configurations describedabove for FIGS. 8, 10 and 11.

A simplified small-scale version of the present invention was tested inthe field using a pulsed EM source and loop antennas for the source andreceiver. The apparatus was configured so as to mimic aborehole-to-surface configuration similar to that of FIG. 11, and asuccessful demonstration of travel time shifting of freely propagatingTEM waves using a small ˜0.5 meter phantom comprised of water loadedwith magnetic nanoparticles. The EM source comprised ˜50 feet of 50-ohmcoax charged to 1000 volts, a 1 mm air spark-gap, and a 3-turn loopantenna 10 cm in diameter in parallel with a 200 pf capacitor. Thisgenerated a 50 ns square wave pulse with a rise time of ˜2 ns,superimposed with a 200 MHz sine wave. The receiver comprised a simple3-turn loop antenna, and the waveforms were recorded with a 4 GHzdigitizing oscilloscope. The signal delay (travel time) was mappedacross the ground surface, while the EM signal from the buried sourcetraversed unmodified sedimentary material, a water-filled phantom and amagnetic nanofluid filled phantom. Using unmodified sediment as abaseline (‘before’ signal), travel time delays consistent with thedielectric and magnetic properties of water and magnetic nanofluid werereadily observed. The results are set forth in FIG. 13.

From the foregoing embodiments it can be seen that the present inventiondirectly visualizes the path of injected fluids in the reservoirenvironment, while generating higher resolution images of the rock andfluid characteristics and distribution in the reservoir.

The present invention thus generates information about the spatialdistribution and composition of fluids in a hydrocarbon reservoir. Sincerock and hydrocarbons generally both have low dielectric constants, lowmagnetic permeability and low electrical conductivity, EM propagationrates are relatively high and cross-well or borehole-to-surface EMtravel-times are relatively short. Thus, water, with high dielectricconstant, often coupled with high conductivity if salty, generates highcontrast in a travel-time map. Injected water, used to displacehydrocarbons, can be imaged with the present invention, and after aperiod of injection, the paths of high permeability and invasion by suchinjected water can be mapped. In a situation where water has beeninjected for a long period of time, it is difficult to differentiateoriginal (connate) water from injected water. By ‘labeling’ newinjection water with magnetic particles, as described, in such a waythat the new injection water imparts magnetic permeability to theinvaded region, it is possible to differentiate new from old water. Thisoccurs since magnetic permeability decreases EM propagation rate andthereby imparts travel-time contrast in the EM velocity images of thereservoir region.

The invention has been sufficiently described so that a person withaverage knowledge in the matter may reproduce and obtain the resultsmentioned in the invention herein Nonetheless, any skilled person in thefield of technique, subject of the invention herein, may carry outmodifications not described in the request herein, to apply thesemodifications to a determined structure, or in the manufacturing processof the same, requires the claimed matter in the following claims; suchstructures shall be covered within the scope of the invention.

It should be noted and understood that there can be improvements andmodifications made of the present invention described in detail abovewithout departing from the spirit or scope of the invention as set forthin the accompanying claims.

What is claimed is:
 1. A method of electromagnetic imaging of asubsurface hydrocarbon reservoir, comprising the steps of: emittingpulses of electromagnetic energy signals to travel through thesubsurface hydrocarbon reservoir; recording waveforms of the emittedelectromagnetic energy signals received at a plurality ofelectromagnetic sensors; forming a measure of the travel time of thesignals received at the plurality of electromagnetic sensors;decomposing the received signals into their frequency components;extracting travel time data of the received signals as a function offrequency; and forming, based on the extracted travel time data as afunction of frequency, an inverted velocity image of subsurface featuresof the subsurface hydrocarbon reservoir.
 2. The method of claim 1,wherein the step of emitting pulses of electromagnetic energy furtherincludes the step of emitting pulses of electromagnetic energy from aplurality of electromagnetic energy sources to travel through thesubsurface hydrocarbon reservoir.
 3. The method of claim 2, furtherincluding the step of lowering the plurality of electromagnetic energysources in a well tool through a well bore in the subsurface reservoir.4. The method of claim 3, further including the step of moving the welltool to a succession of locations in the well bore for emitting pulsesof electromagnetic energy at the locations for travel through thesubsurface hydrocarbon reservoir.
 5. The method of claim 2, furtherincluding the step of locating the plurality of electromagnetic energysources in an array over an earth surface above the subsurfacereservoir.
 6. The method of claim 1, further including the step oflowering the plurality of electromagnetic sensors in a well tool througha well bore in the subsurface reservoir.
 7. The method of claim 6,further including the step of moving the well tool to a succession oflocations in the well bore for forming at the locations a measure of thetravel time of the emitted pulses from the electromagnetic energysource.
 8. The method of claim 1, further including the step of locatingthe plurality of electromagnetic sensors in an array over an earthsurface above the subsurface reservoir.
 9. The method of claim 1,further including the step of performing a tomographic inversionanalysis of the electromagnetic energy sensed by the plurality ofelectromagnetic sensors.